The need for endoprostheses (prostheses that are implantable) in human joints may arise from degeneration of the joint due to disease or wear, or as the result of fracture of one or more bones forming the joint. The endoprosthesis replaces one or more elements of the joint, frequently providing an artificial surface to bear against another element of the joint, or an element of the prosthesis.
Because many endoprosthesis designs involve surfaces that articulate with respect to each other, these surfaces are subject to wear, i.e., to removal of material from the surfaces as the result of contact with, and movement with respect to, other surfaces in the joint. This wear can generate material known as “wear debris,” which are small particles of material that, in some cases cause health problems if released into the body. Some wear debris results from normal contact between surfaces of an articulating endoprosthesis during conditions of use, such as movement between the bearing surfaces of the endoprosthesis. Wear debris can also result from impact between elements of the endoprosthesis, either during normal use conditions, or during more extreme conditions, such as conditions subjecting the joint to unusual movement or shocks. Wear debris can also result from frequent movement of the endoprosthesis beyond its designed range of motion and from multiple components wearing against each other.
Wear debris particles can be found in macrophages (if the debris particles are small enough in size), or in tissue near or around the prosthesis. Inflammatory tissue responses to wear debris (perhaps enabled by inflammatory mediators released by the macrophages) are believed to contribute to bone resorption and some forms of prosthetic loosening, and thus to the resulting need for revision surgery. The amount and type of wear debris generated by various endoprosthesis designs is a parameter that is evaluated in assessing whether such designs should be approved for use.
The degree to which a design may generate wear debris is therefore a parameter that must be balanced against other design considerations for the device. These include materials biocompatibility, mechanical/physical properties, geometry of the endoprosthesis, manufacturing considerations, and the like.
It is also desirable to allow the endoprosthesis to withstand heavy loading while retaining excellent wear and load supporting characteristics, inter alia, in order to make the design more suitable for use in the lumbar region of the spine, where loads are significantly higher than in the cervical spine.